The Federal Metropolitan Commission of the Megalopolis in Mexico City recently ordered more than 2 million cars to remain at home each day to help tackle the air pollution crisis in the city. High levels of air pollution affecting Mexico City since mid March 2016 have led the authorities to declare the first pollution emergency in more than a decade. The World Health Organisation (WHO) recommends that levels of ozone in the air should not exceed 100 micrograms per cubic metre. These limits had been breached several times in recent weeks, however. Once levels of ozone in the city were measured at 160 micrograms per cubic metre, the Environmental Commission issued an emergency order.